JayBee-Keep in mind you can under gear a motor just as bad as over gearing it. Both will cause heat and a lack of performance. When I ran the 48 pitch gears on my XXXS I always ran the orange 90 tooth spur. Just like now with the 64 pitch, the 128 is the biggest you can get. What I usualy do is find a good final drive ratio usually between 6.30 on longer tracks to 7.50 on tighter tracks. This still depends on the motor but should get you in the ballpark. Monster Jr. I have been running around a 6.5 on our med track and a 6.30 with my Core or Hellfire's on the same track. It seems I normally go down 1 to 2 teeth when I run the Monsters over my Core's. Hope I have helped.
Brant